1. Field of the Invention
This invention relates to an electrochromic element which utilizes an electrochemical phenomenon of color formation and extinction, or electrochromic phenomenon.
The electrochromic phenomenon is one in which a substance is colored through the oxidation-reduction reaction when a voltage is applied to it. The electrochemical color forming and extinguishing element may have its applications to, for example, a numerical display element, an X-Y matrix display, an optical shutter, an aperture mechanism, and so on. This electrochromic element can be broadly classified, in terms of its constituent material, into a liquid type and a solid type. The electrochromic element of the present invention is concerned with a full solid type.
2. Description of the Prior Art
FIG. 1 of the accompanying drawing illustrates a conventional full solid type electrochromic element utilizing the electrochromic phenomenon.
The electrochromic element shown in FIG. 1 is constructed with sequential lamination, on a transparent substrate 1, of a first electrode 2 made of a transparent, electrically conductive film, an electrochromic layer 3 as a color forming layer at the cathode side, an insulative layer 4 made of a dielectric film, and a second electrode 5 made of an electrically conductive film.
In the above-described construction of the electrochromic element, the transparent substrate 1 is generally made of a glass plate, although the material is not limited to the glass plate alone, but any other transparent materials such as a plastic (e.g., acrylics, etc.) plate may be used. As to the position of this transparent substrate 1, it may be placed on the second electrode 5, not beneath the first electrode 2, or it may be provided on both surfaces of the first and second electrodes 2, 5 depending on purpose (such as, for example, making it a protective cover for the element). Depending on cases, however, it is necessary that the second electrode be made of a transparent, electrically conductive film, or the electrodes at both sides be made of a transparent, electrically conductive film.
In the following, representative examples of the material to be used generally for the above-mentioned full solid type electrochromic element will be enumerated: the transparent, electrically conductive film to form the first electrode 2 is an indium-tin oxide (ITO) film (containing 5% of SnO.sub.2 in In.sub.2 O.sub.3), and others; the electrochromic layer 3 as the cathode side color forming layer is formed by use of tungsten dioxide (WO.sub.2), tungsten trioxide (WO.sub.3), molybdenum dioxide (MoO.sub.2), molybdenum trioxide (MoO.sub.3), vanadium pentoxide (V.sub.2 O.sub.5), and so forth; the insulation layer 4 as the dielectric film is made of oxide represented by zirconium dioxide (ZrO.sub.2), silicon monoxide (SiO), silicon dioxide (SiO.sub.2), tantalum pentoxide (Ta.sub.2 O.sub.5), and so on, or fluorides represented by lithium fluoride (LiF), magnesium fluoride (MgF.sub.2), and so forth; and the second electrode 5 is made of, for example, a semi-transparent, electrically conductive film of gold.
The full solid type electrochromic element having the above-described construction brings about an electrochemical reaction by application of a voltage across the first and second electrodes 2, 5 thereby forming or extinguishing color. The color forming mechanism in this electrochromic element is generally said to be ascribable to, for example, formation of bronze due to the double injection of cation and electron into the electrochromic layer 3. For instance, when tungsten trioxide (WO.sub.3) is used as the electrochromic substance, there takes place an oxidation-reduction reaction represented by the following equation (1) to form color. EQU WO.sub.3 +xH.sup.+ +xe.sup.- .revreaction.H.sub.x WO.sub.3 ( 1)
Here, in accordance with the equation (1), tungsten bronze H.sub.x WO.sub.3 is produced to form color, and, when the polarity of this applied voltage is reversed, the color is extinguished.
The full solid type electrochromic element of such construction has various disadvantages such that no intended optical density can be obtained at an adequate response speed, and others.